The principle on which a common fluorescent lamp emits light is briefly stated as follows. Referring to FIG. 1, a glass light tube 100 defines a closed space therein, and the closed space is filled with an inert gas, such as argon (Ar), and a small amount of mercury (Hg). The inner wall of the light tube 100 is coated with a phosphor layer. The two ends of the light tube 100 are provided with filaments 101, 102 respectively. Each filament 101, 102, basically a tungsten filament normally coated by BaCO3, has one end connected to a starter 11 and the other end connected to an alternating current (AC) power source 16 by way of a ballast 14 and a switch 15. The starter 11 is composed essentially of a capacitor 12 and a small neon lamp 13 connected in parallel. The small neon lamp 13 is filled with neon (Ne) and provided with two electrodes 131, 132 which form an open circuit during the OFF period. The electrode 132 is made of a bimetal strip. When heated, the electrode 132 curves and contacts with the other electrode 131 to form a short circuit. The ballast 14 is an inductor working in conjunction with the AC power source 16 and the starter 11 and is configured to control the current flowing through the starter 11 and the light tube 100.
As shown in FIG. 1, when the switch 15 is turned on, the neon between the two electrodes 131, 132 of the small neon lamp 13 is discharged and thus generates heat, which causes the bimetal electrode 132 to curve. The curved electrode 132 makes electrical contact with the other electrode 131 and thereby forms a short circuit. In consequence, the current in the small neon lamp 13 runs through and heats the filaments 101, 102, and the heated filaments 101, 102 generate a large amount of hot electrons. As the starter 11 is now in the short-circuit state, there is no voltage across the light tube 100. The short circuit between the two electrodes 131, 132 also cuts off the voltage applied to the neon between the electrodes 131, 132, so the neon is no more discharged and stops generating heat. As a result, the bimetal electrode 132 begins to cool and separates from the other electrode 131, thus instantly terminating the current to the starter 11; the filaments 101, 102; and the ballast 14. Due to the abrupt interruption of current in the ballast (inductor) 14, an oscillation takes place and generates a high voltage at the capacitor 12. The high voltage ionizes the argon in the light tube 100 into argon ions and free electrons, which are accelerated by the externally applied voltage and hit the liquid mercury dispersed in the light tube 100; thus, the liquid mercury is vaporized. While the mercury vapor is hit by the high-speed electrons, the electrons in the mercury atoms undergo energy level transition and generate ultraviolet light. The ultraviolet light, upon striking the phosphor powder on the inner wall of the light tube 100, is converted into visible light. Once the light tube 100 is lit, the inductor 14 serves as an important component for limiting the lamp current and is therefore also known as a ballast or current stabilizer. This type of ballasts are the most typical ballast circuits for fluorescent lamps since the invention of fluorescent lighting and are configured to light fluorescent light tubes at the mains frequency. To sum up, a ballast circuit lights up a fluorescent light tube mainly by the following steps:
1. preheating the filaments at both ends of the fluorescent light tube;
2. generating a high voltage to ionize the argon in the light tube; and
3. stabilizing or limiting the lamp current in the light tube once the light tube is lit.
However, with the advent of the electronic era, it has been found that the lighting efficiency of a fluorescent light tube can be effectively increased by lighting the light tube at a frequency of a few tens kHz. Therefore, in recent years, various electronic ballast circuits have been developed for fluorescent lamps and widely used in fluorescent lighting fixtures. As such, electronic ballast circuits have gradually replace magnetic ballast circuits, which are composed mainly of silicon steel plates and have such disadvantages as bulkiness, heavy weight, and a short starter service life.
Of course, an electronic ballast circuit must also follow the foregoing steps when lighting a fluorescent light tube, although the method for preheating the filaments may vary. The preheating techniques, and whether they are carried out adequately, are an important factor that influences the service life of light tubes. A description of two common preheating methods and their drawbacks follows.    1. PTC preheat-type electronic ballasts: Referring to FIG. 2, the basic structure of this type of electronic ballasts is a half-bridge LC resonant circuit, wherein a positive temperature coefficient (PTC) resistor RPTC is connected in parallel to a resonant capacitor Cr. A PTC resistor is characterized in that its resistance is only a few ohms at normal temperature but can rise instantly to several megaohms when heated by a current flowing therethrough. Therefore, when the ballast is just turned on, the low resistance of the PTC resistor RPTC allows a relatively large current to flow through and thereby heat the filaments at both ends of the light tube 100. As the PTC resistor RPTC itself is heated at the same time, its resistance soon rises to several megaohms, thereby turning the PTC resistor RPTC into a substantially open circuit. During this process, the oscillation voltage of the LC resonant circuit (composed of the resonant capacitor Cr and a resonant inductor Lr) is gradually increased, and a voltage high enough to ionize argon is eventually generated at the resonant capacitor Cr to light the light tube 100. However, as the properties of mass-produced PTC resistors are inconsistent, it is not uncommon that a light tube is lit when its filaments are not yet sufficiently preheated. Should that happen, the argon atoms are very likely to hit and thus damage the filaments, causing the filaments to break prematurely. Because of that, a PTC preheat-type electronic ballast generally only allows a fluorescent light tube to be lit approximately 5000 times. Besides, as PTC components are in a high-temperature (100˜130° C.) state for a long time, the resultant power loss is at least about 0.5 W to 1 W; in consequence, the overall performance of PTC preheat-type electronic ballasts is significantly compromised.    2. Variable-frequency preheat-type electronic ballasts: Referring to FIG. 3, the basic structure of a variable-frequency preheat-type electronic ballast is still a half-bridge LC resonant circuit, wherein a resonant capacitor Cr and a resonant inductor Lr constitute the resonant circuit. The preheating process of such an electronic ballast is carried out by a sweep-frequency driving circuit 110 configured to generate a sweep-frequency signal which goes from a high level to a low level and which is used to drive the half-bridge LC resonant circuit. There are quite a few ICs on the market that are designed specifically for this preheating process, such as Philips UBA2021, International Rectifier IR 2156, and so on. The preheating and starting principle employed by the ballast shown in FIG. 3 is as follows. Soon after the ballast is turned on, the sweep-frequency driving circuit 110 generates a driving signal whose frequency is higher than the resonant frequency of the resonant circuit, thus allowing the current flowing through the resonant capacitor Cr to preheat the filaments. At this moment, the voltage at the resonant capacitor Cr is not high enough to light the light tube 100. As the driving signal generated by the sweep-frequency driving circuit 110 approaches the resonant frequency of the resonant circuit, the filament current running through the resonant capacitor Cr is gradually reduced, but the voltage at the resonant capacitor Cr becomes higher and higher and eventually lights up the light tube 100. Nevertheless, there is a blind spot in this design: while the filaments are preheated, the hot electrons generated by the filaments are not accumulated around the filaments but are dispersed in the light tube 100 due to the increasing AC voltage at the resonant capacitor Cr. In consequence, there tends to be a measurable glow current in the light tube 100 during the preheat period in which the light tube 100 is not yet lit, as shown in FIG. 4. The glow current, resulting from argon ions hitting the filaments, dislodges the electronic powder coated on the filaments and is hard to control. A feasible approach to reduce the glow current is to lengthen the duration of the high-frequency driving signal, as is adopted by the FAN7710V control IC of FAIRCHILD. However, given the Commission Regulation (EC) No 244/2009 that requires the light tube 100 be lit within one second, whether the FAN7710V control IC is up to the task is yet to be determined.
Therefore, the issue to be addressed by the present invention is to design a ballast circuit which applies substantially no voltage to the light tube while preheating the filaments, which enables the hot electrons generated by the preheated filaments to envelop the filaments before the light tube is lit, and which therefore effectively prevents the filaments from being hit by argon ions when the light tube is lit. Thus, the ballast circuit proposed by the present invention not only ensures that the electronic powder coated on the filaments will not be easily lost, but also significantly increases the service life of the fluorescent light tube.